Window treatments are commonly used to enhance the appearance of a window area. There have been a myriad of window treatment concepts and devices that have been proposed, and to some extent utilized. These devices are primarily for a traditional window that is flush with the wall that it is mounted within. Thus, perpendicular angles for cornice designs are appropriate and even desirable. However, there are many applications, such as bay windows and arched windows that require non-perpendicular angles. Thus, there exists a present and continuing need for a window treatment joint that is simple in design, inexpensive in construction, and amenable to use in association with a wide variety of window shapes, angles, and designs.